Fallback to lowercased kinds for empty singulars

[nolar]

K3s has all builtins with empty singulars (reasons of this are not known). To still match the selectors that use singular=... or any_name=... with a singular value, fall back to lowercased kinds (in the assumption that they are the same as singulars, but CamelCased).

$ z get --raw /api/v1 | jq
………
    {
      "name": "pods",
      "singularName": "",          <<<<<<<<<<<
      "namespaced": true,
      "kind": "Pod",
      "verbs": [
        "create",
        "delete",
        "deletecollection",
        "get",
        "list",
        "patch",
        "update",
        "watch"
      ],
      "shortNames": [
        "po"
      ],
      "categories": [
        "all"
      ],
      "storageVersionHash": "xPOwRZ+Yhw8="
    },
………
    {
      "name": "services",
      "singularName": "",          <<<<<<<<<<<
      "namespaced": true,
      "kind": "Service",
      "verbs": [
        "create",
        "delete",
        "get",
        "list",
        "patch",
        "update",
        "watch"
      ],
      "shortNames": [
        "svc"
      ],
      "categories": [
        "all"
      ],
      "storageVersionHash": "0/CO1lhkEBI="
    },
………

A follow-up for #600.